Iemp gauge erratic

Theres a bit of a story behind this one,so bear with me.
Last May whilst in the Pyrenees my 300tdi started running a bit hot,not off the scale,just touching the red area on the gauge.
So me thinks,thermostat is Fubared,so I took it out.All good,once warmed up gauge reads normal.
I have run it al the rest of this year with the thermostat minus the middle which I punched out.I read somewhere it's not good to run totaly without a stat.
Now the werather is much colder,I thiught I'd replace the Thermostat.So,motor warms up nice & quickly now,but not long after reaching normal operating temp it rises to just under the red.If I turn the lights or fan on the temp gauge goes right up into the red immediately.
So, I'm thinking something electrical,I have changed the temp sensor & stat.Do you think bad earth,& if so where would you suggest I start looking?
Thanks a lot guys.
Marc.

The gauge works by sending an earth from the sender unit in the head. I'd disconnect the plug there and test again with the lights/fan. If the gauge rises, you have a fault in the wiring back from the sender. If so, most likely behind the dash - look for wiring insulation damage where it passes over the many sharp metal bits.

zeaphod said:

The gauge works by sending an earth from the sender unit in the head. I'd disconnect the plug there and test again with the lights/fan. If the gauge rises, you have a fault in the wiring back from the sender. If so, most likely behind the dash - look for wiring insulation damage where it passes over the many sharp metal bits.

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I'd not thougt of that.
I'll try it later today.
Thanks
Marc

Check your earths

Just a quick update on this problem.
Turned out to be the earth strap from the battery to the chassis/transfer box.
Cleaned it up & re fitted,up to yet gauge has not gone above normal/vertical.
Thanks for the ideas & all your imput fellas.Much aprechiated.

Marc

Pleased to hear you got it sorted.
Remember that many problems are caused by poor earthing!